1. Field of the Invention
The present invention relates to a voltage-controlled resonator that can be used in a high-frequency filter and oscillator circuit, a method of fabricating the same, a method of tuning the same, and a mobile communication apparatus.
2. Description of the Related Art
In a voltage-controlled resonator, it is important to maintain a required resonant frequency. Furthermore, the need to reduce the size and cost of voltage-controlled resonators has been growing in recent years.
A voltage-controlled resonator such as described in U.S. Pat. No. 5,475,350 has been known in the prior art. In the case of this resonator, the construction is such that a variable-capacitance element and a circuit pattern formed on the upper surface of a dielectric forming a resonant circuit are exposed, i.e., no shield function is incorporated in the structure. Accordingly, this prior art resonator has had the shortcoming that because of its poor shielding properties, its resonant frequency is shifted (deviated) from the design value when a dielectric substance is brought close to it. To overcome this shortcoming, an arrangement such as that shown in FIG. 5(C) has been devised in which, after mounting the voltage-controlled resonator on an apparatus circuit board 310, a shield case 309 is mounted covering the entire apparatus circuit board 310.
Referring now to FIGS. 5(A) to 5(C), one example of the above prior art voltage-controlled resonator will be described.
FIG. 5(A) is a perspective view of the prior art voltage-controlled resonator, and FIG. 5(B) is a side cross-sectional view of FIG. 5(A). FIG. 5(C) is a side cross-sectional view showing the condition in which the shield case 309 is mounted covering the entire apparatus circuit board 310 on which the voltage-controlled resonator of FIG. 5(A) has been mounted.
In FIGS. 5(A) and 5(B), reference numeral 301 is a dielectric where a resonant circuit is formed, 302 is an internal electrode of the dielectric 301, 303 is a circuit pattern formed on the upper surface of the dielectric 301, 304 is a variable-capacitance element mounted on the dielectric 301, 305 are terminal electrodes for connecting an external circuit, and 306 is a tuning pattern formed on the upper surface of the dielectric 301.
The operation of the thus constructed voltage-controlled resonator will be described below.
In the voltage-controlled resonator shown in FIGS. 5(A) to 5(C), the resonant circuit is formed by the internal electrode 302 of the dielectric 301, and the variable-capacitance element 304 is electrically connected to the resonant circuit so that the resonant frequency of the resonant circuit can be changed. The controlling voltage for the variable-capacitance element is supplied from an external circuit through the terminal electrodes 305 and through the circuit pattern 303.
By mounting the shield case 309 over the entire construction after mounting the above voltage-controlled resonator on the apparatus circuit board 310, resonant frequency shifts due to effects from the external circuit can be suppressed.
Mounting the shield case 309, however, has the undesirable side effect of increasing the resonant frequency of the voltage-controlled resonator.
To suppress such a resonant frequency shift resulting from the mounting of the shield case 309, frequency tuning is performed by shaving the tuning pattern 306.
More specifically, in the above case, since the size of the shield case is large, the amount of the frequency shift is also large. When the amount of shift is large, it has been the practice to remove the mounted shield case once again, shave the tuning pattern 306, measure the resonant frequency, and check if it falls within tolerance, in order to bring it close to the design value. If the resonant frequency is still outside the tolerance, the above adjustment work has had to be repeated as many times as necessary until the resonant frequency is brought within the tolerance.
As a measure to keep the amount of the resonant frequency shift as small as possible, the arrangement shown in FIG. 5(D) has been devised in which an individual shield case 311 is mounted over the voltage-controlled resonator to provide the shielding.
While this arrangement has been able to reduce the amount of the resonant frequency shift to some degree, adjustment work similar to that described above has had to be performed, and since the mounting condition changes in a delicate manner each time the shield case is mounted, it has been difficult to precisely tune the resonant frequency.
Besides, the shortcoming of increased height of the voltage-controlled resonator itself has remained unresolved.
A further problem has been that mounting the shield case not only increases the complexity of the fabrication process but also requires that the size of the dielectric 301 be increased, thus increasing the cost.